<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .sky {
            height: 180px;
            background: #007fd5;
            position: relative;
            overflow: hidden;
            -webkit-animation: sky_background 50s ease-out infinite;
            -moz-animation: sky_background 50s ease-out infinite;
            -o-animation: sky_background 50s ease-out infinite;
            animation: sky_background 50s ease-out infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }
        .sky .clouds_one {
            background: url(https://images.cnblogs.com/cnblogs_com/endv/1074042/o_cloud_one.png);
            position: absolute;
            left: 0;
            top: 0;
            height: 100%;
            width: 300%;
            -webkit-animation: cloud_one 50s linear infinite;
            -moz-animation: cloud_one 50s linear infinite;
            -o-animation: cloud_one 50s linear infinite;
            animation: cloud_one 50s linear infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }
        .sky .clouds_two {
            background: url(https://images.cnblogs.com/cnblogs_com/endv/1074042/o_cloud_two.png);
            position: absolute;
            left: 0;
            top: 0;
            height: 100%;
            width: 300%;
            -webkit-animation: cloud_two 75s linear infinite;
            -moz-animation: cloud_two 75s linear infinite;
            -o-animation: cloud_two 75s linear infinite;
            animation: cloud_two 75s linear infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }
        .sky .clouds_three {
            background: url(https://images.cnblogs.com/cnblogs_com/endv/1074042/o_cloud_three.png);
            position: absolute;
            left: 0;
            top: 0;
            height: 100%;
            width: 300%;
            -webkit-animation: cloud_three 100s linear infinite;
            -moz-animation: cloud_three 100s linear infinite;
            -o-animation: cloud_three 100s linear infinite;
            animation: cloud_three 100s linear infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }
        .sky {
            height: 180px;
            background: #007fd5;
            position: relative;
            overflow: hidden;
            -webkit-animation: sky_background 50s ease-out infinite;
            -moz-animation: sky_background 50s ease-out infinite;
            -o-animation: sky_background 50s ease-out infinite;
            animation: sky_background 50s ease-out infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }

        .sky .clouds_one {
            background: url("https://images.cnblogs.com/cnblogs_com/endv/1074042/o_cloud_one.png");
            position: absolute;
            left: 0;
            top: 0;
            height: 100%;
            width: 300%;
            -webkit-animation: cloud_one 50s linear infinite;
            -moz-animation: cloud_one 50s linear infinite;
            -o-animation: cloud_one 50s linear infinite;
            animation: cloud_one 50s linear infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }

        .sky .clouds_two {
            background: url("https://images.cnblogs.com/cnblogs_com/endv/1074042/o_cloud_two.png");
            position: absolute;
            left: 0;
            top: 0;
            height: 100%;
            width: 300%;
            -webkit-animation: cloud_two 75s linear infinite;
            -moz-animation: cloud_two 75s linear infinite;
            -o-animation: cloud_two 75s linear infinite;
            animation: cloud_two 75s linear infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }

        .sky .clouds_three {
            background: url("https://images.cnblogs.com/cnblogs_com/endv/1074042/o_cloud_three.png");
            position: absolute;
            left: 0;
            top: 0;
            height: 100%;
            width: 300%;
            -webkit-animation: cloud_three 100s linear infinite;
            -moz-animation: cloud_three 100s linear infinite;
            -o-animation: cloud_three 100s linear infinite;
            animation: cloud_three 100s linear infinite;
            -webkit-transform: translate3d(0, 0, 0);
            -ms-transform: translate3d(0, 0, 0);
            -o-transform: translate3d(0, 0, 0);
            transform: translate3d(0, 0, 0);
        }

        @-webkit-keyframes sky_background {
            0% {
                background: #007fd5;
                color: #007fd5
            }
            50% {
                background: #000;
                color: #a3d9ff
            }
            100% {
                background: #007fd5;
                color: #007fd5
            }
        }
        @-moz-keyframes sky_background {
            0% {
                background: #007fd5;
                color: #007fd5
            }
            50% {
                background: #000;
                color: #a3d9ff
            }
            100% {
                background: #007fd5;
                color: #007fd5
            }
        }
        @keyframes sky_background {
            0% {
                background: #007fd5;
                color: #007fd5
            }
            50% {
                background: #000;
                color: #a3d9ff
            }
            100% {
                background: #007fd5;
                color: #007fd5
            }
        }

        @-webkit-keyframes cloud_one {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
        @-moz-keyframes cloud_one {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
        @keyframes cloud_one {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }

        @-webkit-keyframes cloud_two {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
        @-moz-keyframes cloud_two {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
        @keyframes cloud_two {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }

        @-webkit-keyframes cloud_three {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
        @-moz-keyframes cloud_three {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
        @keyframes cloud_three {
            0% {
                left: 0
            }
            100% {
                left: -200%
            }
        }
    </style>
</head>
<body>
<div class="sky">
    <div class="clouds_one"></div>
    <div class="clouds_two"></div>
    <div class="clouds_three"></div>
</div>
</body>
</html>